Hybrid internal combustion/electric vehicles or electric vehicles specifically incorporate high-capacity batteries. Likewise, fuel cell-powered vehicles incorporate high-capacity fuel cells. Voltage sources of this type are used to drive an alternating current electric motor via an inverter. The requisite voltage levels for motors of this type are of the order of several hundred volts, typically of the order of 400 volts.
In order to achieve high power and capacity ratings, a number of groups of accumulators are arranged in series. The number of stages (number of groups of accumulators) and the number of accumulators arranged in parallel in each stage will vary as a function of the voltage, current and capacity required for the battery. The combination of a number of accumulators is called an accumulator battery. Electrochemical accumulators used for vehicles of this type are generally of the lithium-ion type, because of their high energy storage capacity, with limited weight and volume. Lithium-ion/iron phosphate (LiFePO4) battery technologies are undergoing substantial developments in the interests of the achievement of a high level of intrinsic safety, at the expense of a slightly reduced energy storage density. An electrochemical accumulator generally has a rated voltage of the following order of magnitude:                3.3 V, in the case of lithium-ion/iron phosphate technology (LiFePO4),        4.2 V, in the case of cobalt oxide-based lithium ion technology.        
High-capacity fuel cells are also envisaged as DC voltage sources for private motor vehicles or public transport vehicles. A fuel cell is an electrochemical device which converts chemical energy directly into electrical energy. The fuel used in a fuel cell is dihydrogen. Dihydrogen is oxidized on one electrode of the cell, and atmospheric dioxygen is reduced on a further electrode of the cell. The resulting chemical reaction produces water. The major advantage of the fuel cell is the avoidance of the generation of any harmful substances at the site of electricity generation.
A fuel cell is comprised of a stack of multiple cells, arranged in series. Each cell generates a voltage of the order of 1 volt, and the stacking thereof permits the generation of a supply voltage of a higher level, for example of the order of one hundred or several hundred volts.
FIG. 1 illustrates an example of vehicle 1 according to the prior art, incorporating a power battery 12. The battery 12 incorporates electrochemical accumulators 121 connected in series, for example between 40 and 150 accumulators, depending upon the voltage required and the type of accumulators used. The battery 12 applies a voltage +Vbat to a first terminal, and a voltage −Vbat to a second terminal. The accumulators 121 are connected in series by means of electrical power connections. The terminals of the battery 12 are connected to a DC interface of an inverter 16. An electric motor 17 is connected to an AC interface of the inverter 16.
The connection between the terminals of the battery 12 and the DC interface of the inverter 16 is formed by a protection circuit 13 and a power coupling circuit 15. The protection circuit 13 comprises the fuses 131 and 132, which are configured for the opening of the connection in the case of a short-circuit. The protection circuit 13 also comprises the disconnectors 133 and 134, which permit the disconnection of the battery 12, thereby ensuring, in a secure and visually confirmable manner, that the battery is in a safe condition in order to conduct operations on the vehicle 1.
A power coupling circuit 15 comprises switches 151 and 152, which permit the selective connection/disconnection of the terminals of the battery 12 to the DC interface of the inverter 16. The opening/closing of the switches 151 and 152 is controlled by a command circuit 18, which is configured in the form of a computer for the monitoring of the operation of the battery 12. The command circuit 18 will only close the switches 151 and 152 when the vehicle is ready for start-up. The switches 151 and 152 may be used to interrupt the power supply to the motor 17 in case of faults. The command circuit 18 is typically powered by a supply battery 191 supplying the on-board system of the vehicle 1, the voltage rating of which is substantially lower than that of the battery 12.
A DC/DC converter, not illustrated, converts the voltage of the battery 12 to the voltage of the on-board system of the vehicle 1, which is generally close to 12 V.
The inverter 16 incorporates 6 transistors of the IGBT type, forming 3 switching arms, and the motor 17 is directly supplied by this inverter 16. A decoupling capacitor 19, rated to a few hundred microfarads, is arranged in parallel with the inverter 16. This capacitor 19 is used for the decoupling of the voltage, in order to minimize fluctuations in the supply voltage associated with the rapid switching of IGBTs upon closing and opening.
The energy stored in the capacitor 19 is: ½ CU2. Accordingly, it is not possible to proceed with the direct closing of the contactors 151 and 152 on the inverter 16, as this will generate an overcurrent of several thousand amperes as a result of the presence of the capacitor 19, initially in the discharged state. An overcurrent of this magnitude might induce a hazardous overvoltage for equipment, as a result of the overvoltage generated on the capacitor 19 and on the IGBTs of the inverter 19 by the stray inductance of cabling.
The vehicle 1 therefore incorporates a precharge system for the limitation of the charging current of the capacitor 19, whereby the contactors 151 and 152 will not close until the capacitor 19 is charged. The contactor 152 is closed in the first instance, with no current flowing. The precharge system is arranged in parallel to the contactor 151, and is comprised of a resistor and a contactor. The resistor, for example a vitrified coiled precharge resistor or a wire wound resistance, will be subject to a sudden load increase upon the charging of the capacitor. This resistor must be capable of storing thermal energy lost in the form of heat, and of dissipating this energy.
In practice, the battery 12 will be subject to variations in voltage during discharging. Moreover, the failure of an accumulator in the battery 12 will entail the necessity for the decoupling thereof from the inverter 16, with the consequent shutdown of the vehicle 1, which will thus be rendered inoperable.
The vehicle marketed by the company Toyota under the brand name “Prius” incorporates a step-up/step-down voltage converter which applies a stabilized voltage to the input of the inverter, generated from the fluctuating voltage delivered by the power battery. The converter operates in step-up mode when the power battery is supplying the inverter, and in step-down mode when the electric motor is operating as a generator. In this vehicle, the power battery is only used for periods of relatively brief duration, controlled by a computer, as the combustion engine fitted to the vehicle accounts for the greater proportion of the operating cycle. Accordingly, the computer can control both the intervals and the duration of phases for the operation of the converter. Such a converter has thus high cooling requirements and, in consequence, the associated cost thereof, are relatively modest as a result.
The adoption of a converter of this type in a vehicle provided with an electric motor, but with no combustion engine, is problematic. In practice, in a vehicle of this type, the converter will be in service throughout the entire operating cycle of the electric motor. In consequence, particular attention will need to be paid to the effective design of the cooling system for the converter, thereby necessitating a converter which is both cumbersome and expensive, and which may occupy approximately one third of the volume of the power battery, in the light of the power level which is effectively deployed in the battery.
Moreover, in certain applications, the capacity deployed exceed 100 kWh. For example, the following capacities may be deployed in storage batteries: 200 kWh for the propulsion of a bus, between 100 kWh and several MWh for a storage facility associated with a renewable energy generator, and over 100 kWh for a standby storage facility in thermal power plants or IT complexes. Given the dimensions and mass of batteries of this type, the latter are generally comprised of independent modules which are interconnected in series and/or in parallel, thereby facilitating the manufacture and transport thereof. In power batteries of this type, the conduct of maintenance operations by the isolation of one or more modules, whilst maintaining continuity of service, is particularly problematic. The various modules will show different characteristics (associated with differences in their charging status, variations in production, age or wear) and, accordingly, cannot be connected directly together by means of simple contactors. The achievement of an equal level of charging in all modules prior to the connection thereof directly is, at best, a complex operation; at worst, an impossible operation, particularly during the operation of the power battery. In consequence, modules of this type are provided with respective power converters for the adjustment of their output voltage, thereby permitting the interconnection thereof. Converters of this type are particularly expensive, cumbersome and complex. Moreover, converters of this type must incorporate components which are resistant to the high levels of power deployed, resulting in substantial conduction and switching losses, and also restricting the frequency of switching to exceptionally low levels.